According to the state of the art, e.g., DE-PS 974,418 one compressor impeller is arranged overhung on each pinion shaft on one side or on both sides in the geared multishaft turbocompressors. Connecting pipelines are located between the stages.
The gas enters the impeller axially via the suction housing and is slowed down in the spiral housing. The external diameter of the impellers steadily decreases with increasing compression and consequently with decreasing suction volume flow rate of the compressor stages to maintain optimal volume flow rates, and the speeds of the pinion shafts steadily increase to maintain the circumferential velocity of the impellers necessary for the stage compression ratio in question. The maximum diameter of the central gear, is determined by its maximum circumferential velocity, this leads to steadily decreasing pinion diameters and with it an increase in the number of revolutions and to steadily decreasing numbers of pinion teeth.
When the limit tooth number is reached, it is necessary to insert intermediate gears to further increase the speed. This leads to additional mechanical frictional losses in the gear bearings and gear teeth.
Rotor dynamic problems in terms of vibration stability, etc., may also occur at the high speeds.
The solution described in German Offenlegungsschrift No. DE-OS 25,15,628, involves one pair of impellers arranged on each pinion shaft back to back on one side only on the side of the gear facing away from the drive. However, this leads to impairments rather than to improvements from the viewpoint of rotor dynamics. This is particularly true because the overhanging rotor part is at a great distance from the center of gravity in this arrangement due to the radial suction hole arranged between the gearbox case and the impeller.
An intercooler, which cools the gas approximately to the initial temperature of compression, is normally arranged between the individual compressor stages. As a result, the final temperatures of the individual compressor stages are also correspondingly low, corresponding to the increase in temperature in the stage.
If the process also requires a high final temperature, the final stage must operate at a correspondingly high circumferential velocity to reach the final temperature required. This causes a further increase in the speed of the pinion shaft, as a result of which the above-mentioned problems are further exacerbated.
A way out to avoid the high circumferential velocity would be to increase the pressure number of the stage, e.g., by setting steeper impeller discharge angles, but this impairs the characteristics concerning the surge limit of the turbocompressor.
Another possibility would be the series connection of two stages with a connecting pipeline without an intercooler. Besides the extra cost for a second pinion shaft end and two complete spiral stages, this leads to additional flow losses because of the double conversion of pressure energy and velocity energy, additional leakage losses at the outlet of the pinion shaft from the spiral housing, and mechanical frictional losses.